A spin valve or a giant magnetoresistive (GMR) sensor detects magnetic field signals through the resistance changes of a read element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the read element. A conventional magnetoresistive sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varies as the square of the cosine of the angle between the magnetization in the read element and the direction of sense current flow through the read element. Recorded data can be read from a magnetic medium, because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in magnetoresistive ratio (ΔR/R) in the read element.
A spin valve or a GMR sensor has been identified in which the resistance between two uncoupled ferromagnetic layers is observed to vary as the cosine of the angle between the magnetizations of the two layers and is independent of the direction of current flow. The latter spin valve produces a magnetoresistance that, for selected combinations of materials, is greater in magnitude than the AMR. Typically, the higher GMR ratio (ΔR/R) results in higher amplitude and better overall performance of the spin valve recording heads.
Typically, a conventional spin valve includes a ferromagnetic free layer, a ferromagnetic pinned layer, which is exchange-coupled with an antiferromagnetic (AF) layer, and a spacer layer located between the free layer and the pinned layer. Often an underlayer of metal, such as Ta, Zr, and Cu, or metal oxide, such as NiO and NiMnOx, is applied to enhance the ΔR/R ratio of the spin valve. Antiferromagnetic layers shift the hysteresis loops of ferromagnetic films away from the zero field axis. The shift brings the most sensitive part of the magnetoresistive loop close to zero field.
However, the underlayers of spin valves made by the prior art do not optimize the ΔR/R ratio of the spin valve. Furthermore, the spin valves of the prior art do not optimize the pinning strength, which is the external field applied to a spin valve enough to unpin the magnetization of the pinned layer. Typically, the higher the pinning field is, the better is the spin valve's performance. The desirable value of pinning strength is typically above 400 Oe. The same mechanisms that increase spin valve ΔR/R often lower its pinning strength Hua. Spin valves of the prior art cannot balance high values of ΔR/R and high pinning strength to improve the performance of spin valves.
There is a need, therefore, for a spin valve structure that improves spin valve magnetic properties.